Dropout fuse assembly including improved current limiting fuse structures

ABSTRACT

A current limiting fuse structure including a latch tripping means or device disposed within the housing of said fuse structure or assembly. The latch tripping device comprises a spring-loaded plunger which when actuated moves through an end ferrule of the fuse housing to thereby trip a latch in the fuse assembly which normally latches the fuse housing in place in one of the fuse assembly terminals. Two operating conditions must both exist simultaneously before the plunger is actuated to unlatch the fuse housing. First, a portion of current sensitive fusible wire disposed in the tripping device to prevent the plunger therein from moving into a tripping position must be blown to thereby allow the plunger to move, and second, the current flowing through the fuse must be substantially zero so that no arc is maintained between the fuse housing ferrule as it swings or moves away from the latching assembly and the stationary part of the overall fuse assembly. This is done by providing an electromagnet in series circuit relationship with the main fuse element so that when the current through the main fuse element is zero, the current through the electromagnet is also zero, and, consequently, an override means which normally prevents the plunger from moving into a position to disengage the latch is actuated. The override means may comprise a springloaded shutter which is held in a first position by the electromagnet when current flows through the electromagnet, but which moves to a second position when the current ceases to flow through the electromagnet thus freeing the plunger to engage in an unlatching operation. In another example, the electromagnet may cause contacts to close when deenergized. These contacts are connected in series circuit relationship with the previously described fusible wire which, in turn, holds the plunger in place. When the contacts are closed, the fusible wire blows or melts, thus freeing the plunger for an unlatching operation.

United States Patent 1 Blewitt DROPOUT FUSE ASSEMBLY INCLUDING IMPROVED CURRENT LIMITING FUSE STRUCTURES [75] Inventor: Donald D. Blewitt, Pittsburgh, Pa.

[.73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Mar. 22, 1973 [21] v Appl. No.: 344,015

[52] U.S. Cl.....- 337/175, 337/150, 337/174,

[51] Int. Cl. H01h 71/10 [58] Field of Search 337/150, 174, 175, 176,

[56 References Cited UNITED STATES PATENTS 2,843,704 7/1958 Wood 337/175 2,655,576 10/1953 Milne et a1 337/174 X 3,401,247 9/1968 Upton et al..... 337/221 3,447,114 5/1969 Frink et a]. 337/171 3,094,597 6/1970 Barta 337/217 X Primary ExaminerBemard A. Gilheany Assistant Examiner-F. E. Bell Attorney, Agent, or FirmW. A. Elchik [5 7] ABSTRACT A current limiting fuse structure'including a latchtripping means or device disposed within the housing of said fuse structure or assembly. The latch tripping device comprises a spring-loaded plunger which when actuated moves through an end ferrule of the fuse housing to thereby trip a latch in the fuse assembly which normally latches the fuse housing in place in one of the fuse assembly terminals. Two operating conditions must both exist simultaneously before the plunger is actuated to unlatch the fuse housing. First, a portion of current sensitive fusible wire disposed in the tripping device to prevent the plunger therein from moving into a tripping position must be blown to thereby allow the plunger to move, and second, the current flowing through the fuse must be substantially zero so that no arc is maintained between the fuse housing ferrule as it swings or moves away from the latching assembly and the stationary part of the overall fuse assembly. This is done by providing an electromagnet in series circuit relationship with the main fuse element so that when the current through the main fuse element is zero, the current through the electromagnet is also zero, and, consequently, an override means which normally prevents the plunger from moving into a position to disengage the latch is actuated. The override means may comprise a spring-loaded shutter which is held in a first position by the electromagnet when current flows through the electromagnet, but which moves to a second position when the 10 Claims, 4 Drawing Figures PRIOR ART PAIENTEDAPR 21914 3.801.946 sum 2 0? 2 PRIOR m- FIG. 2.

DROPOUT FUSE ASSEMBLY INCLUDING IMPROVED CURRENT LIMITING FUSE STRUCTURES BACKGROUND OF THE INVENTION This invention is related to automatic latch opening means for a dropout fuse assembly and it has particular relationship to those portions of automatic latch opening means which are disposed within the fuse housing.

High voltage dropout fuse assemblies often include a latching mechanism in one portion thereof wherein a portion of the end ferrule of a fuse housing is held in place by a latching mechanism in the stationary portion of the overall dropout fuse assembly. Means are usually provided within the fuse housing or barrel to actuate a plunger or similar mechanism to protrude or project through the end of the fuse housing when a fusing or interrupting operation has occurred to open the previously described latch thereby causing the fuse unit to move away from the latching mechanism such as by pivoting away with the aid of gravity. Lzatching mechanisms and tripping means of the type previously described are disclosed in US. Pat. Nos. 3,401,247 issued to C. W. Upton, Jr. et al. and 3,447,l14 issued to R. E. Frink et a]. which are both assigned to the same assignee as the present invention. One of theproblems that arises with respect to the apparatus of the previously mentioned patents lies in the fact that the actuating or unlatching mechanism may cause the fuse housing to swing away or fall away from the stationary latch terminal before the current flowing in the fuse element has decreased tozero. A dropout power fuse assembly may have a rating of 69 RV for example. Consequently, an electrical arc may be drawn between the moving ferrule and the stationary latching mechanism. The presence of an electrical arc under that condition is undesirable and it is therefore advantageous to provide a fuse housing assembly in which the fuse housing will not swing away from the stationary portion of the assembly before the electrical current in the fuse has decreased to substantially zero amperes.

SUMMARY OF THE INVENTION In accordance with the invention, a latch actuating or tripping-mechanism is disclosed which will not become operable until both of two conditions have occurred. First, the mainfuse element must at least have begun to have been blown and second the current flowing through the fuse must be reduced to substantially zero. In order to accomplish this, a section of fusible wire of sufficient tensile strength is utilized to hold a spring loaded plunger in a charged position. The fusible wire is disposed in parallel electrical circuit relationship with the main fuse element of the fuse. In addition, a second means including an electromagnet is disposed in series electrical circuit relationship with the main fuse element. The electromagnet may be disposed to normally maintain a spring loaded shutter over an opening through which the previously described plunger must move in order to accomplish the necessary unlatching operation or the electromagnet may maintain a set of electrical contacts which are adapted to electrically connect the previously described fusible wire in parallel circuit relationship with the main fuse element in an open state until deenergized. As long as current is flowing through the main fuse element and is not of sufficient value to cause the main fuse element to blow, the electromagnet will prevent actuation of the plunger to an unlatching position. However, if the current flowing through the main fuse element increases to a value sufficient to cause the main element of the fuse to blow or melt, the plunger will nevertheless remain in a charged position until the current in the main fuse element goes to zero. This is because the electromagnet will maintain either the previously described shutter in a blocking position or the previously described contacts in an open state, thus preventing residual current from flowing into the plunger wire to melt or blow it.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, reference may be had to the preferred embodiments, exemplary of the invention shown in the accompanying drawings in which:

FIG. 1 shows a dropout fuse assembly in which the disclosed latch actuating mechanism may be utilized;

FIG. 2 shows a prior art latch actuating mechanism;

FIG. 3 shows one embodiment of the present invention in which electrical contacts are used to assist in actuating the plunger of the deactuating mechanism; and

FIG. 4 shows another embodiment of the invention in which a magnetic shutter is used to assist in actuating the plunger to a deactuating position.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and FIG. 1 in particular, a high voltage dropout, power fuse structure or assembly 10 is shown. Assembly 10 includes standoff insulators 12 and 14 which support electrically conducting terminal means 16 and 18, respectively. Terminal means 18 may comprise a support member for a fuse unit or housing assembly 19. Terminal member 16 may comprise a latching mechanism for fuse housing assembly 19. Consequently, when a fuse housing assembly 19 is in a conducting position, electrical current may flow between electrically conducting terminal portions 16 and 18 through a fuse element (not shown) in the fuse housing 19. However, when an overload current is present, the fuse element in fuse housing assembly 19 may blow. In this case, it is desirable after the fuse has blown to disengage the fuse housing assembly 19 from one of the terminals so that a path of electrical conductance which may result along the main body 20 of the fuse housing assembly 19 cannot conduct electrical current thus defeating the purpose of the fuse to provide an open circuit. In this particular dropout fuse assembly l0, fuse housing assembly 19 will pivot or rotate in a direction 21 about pivot 22 thus moving the electrically conducting end ferrule 24 away from the combination latching mechanism and terminal 16. End ferrule 25 remains in contact with support assembly 18. Consequently, an electrically insulating gap between latching assembly and terminal 16 and support assembly 18 is created or results which prevents electrical conduction under even relatively high voltage stress.

Referring now to FIG. 2, a portion of the previously mentioned prior art fuse housing assembly 19 is shown. Fuse housing assembly 19 comprises a main body 20 which may be formed from an electrically insulating, tubular material in which various portions of the fuse assembly may be disposed. An electrically conducting end ferrule 24 is disposed at the upper end of the fuse housing 20. End ferrule 24 is constructed to include protrusion 28 whichmay be the means whereby the entire fuse assembly 19 is latched or retained by the latching assembly and terminal 16 of FIG. 1. End ferrule 24 also includes an upper portion 36 in which a hole 38 is provided. Within the end ferrule 24 which may be hollow and which may communicate with the interior hollow portion of housing is disposed a plunger mechanism or latch tripping means or device 40 which includes plunger member 42 which is disposed in a housing 41 and which is spring-loaded by a biasing spring 44 at the base thereof to move in a direction 45 when free to protrude or. project through the hole 38 into the tripping portion of the latching assembly terminal 16, as shown in FIG. 1, thus causing an actuation or release of the latch in the latch terminal assembly 16 and subsequently causing disengagement of the ferrule 24 from theterminal means 16. However, plunger 42 is maintained in the charged state by the action of a section of fusible material or wire 50 which is disposed to pass through ahole 52 in the end of plunger 42 to constrain plunger 42 from moving in the direction 45.'The high strength fusible wire 50 is held in place by an electrically insulating disk or member 54 against which the spring 44 exerts a force. Disposed adjacent the end ferrule 24 is the main element of fusible material 53,- such as an alloy of tinor silver which is connected to end ferrule 24 at an electrically conducting junction 55. Also connected to electrically conducting end ferrule or end terminal 34 at a junction point such as 56 is wire 50. The other end of the wire 50 and the other end of the main fuse element 53 may be joined or connected at a common junction point 58 so that the auxiliary fuse wire 50 is in parallel circuit relationship with the main fuse element 53. The junction point 58 is electrically connected to another end terminal, such as shown in FIG. 1 to complete a circuit between the end ferrules of the fuse assembly housing 19. In high voltage applications, the wire 50 will conduct sufficient electrical current to fuse or thermally disintegrate as soon .as main fuse element 53 has begun to fuse even though electrical current continues to flow through thatportion of the fuse housing 20 which previously contained the main fuse element 53. As a result, although high electrical current may still continue to flow, the

'plunger 42 will have begun to move in a direction 45 by the discharging force of the spring 44 to subsequently disengage the fuse terminal 24 from the previously described latching mechanism. If the current has not been completely interrupted or reduced to zero by the time the dropout motion as indicated by 21 in FIG.

1 has started, an arc may be drawn or sustained between the terminal 24 and that portion of the fuse assembly to which it had been previously connected.

Referring now to FIG. 3, an embodiment of the present invention is shown which includes a plunger 42 disposed within terminal or end ferrule 24 which is, in

her 54 is disposed in the bottom of plunger mechanism housing 40 to maintain the wire 50 and the spring 44 in place and to function as a spring seat. An electromagnet 62 having an electrically conducting and insulated winding 64 having one or more conductor turns is disposed adjacent the end terminal 24 at the junction point 65. The electromagnet 62 which may have a magnetic core 66 has the other end of the electrically conducting winding 64 joined or connected to a wire 68 at junction point 70. Junction point 70 is also connected to the top or one side of the electrically conducting main fuse element 53. A pair of electrically conducting contacts 72 and 74 which are separable and one of which is spring-loaded by way of a spring 76 which has the other end thereof secured to the housing 20 at a point 78. The contacts 72 and 74 when actuated to a closed position provide a series circuit connection between the wire 68 and the wire 50. The wire 50 is electrically connected to the other end of the main fuse element 53 at a junction point 58 which is electrically connected to the other terminal or ferrule (not shown) of the fuse unit 19.

In operation, when electrical current flows through the terminal 24 into the junction 65 of the insulated lead of the electromagnet 62, a magnetic force is applied to contact or terminal 72 thus actuating that terminal to an open position and charging the spring 76. As long as the current flowing through the coil of the electromagnet 62 is not sufficient to blow or fuse main fuse element 53, substantially no electrical current is provided to the wire 50 to thereby cause movement of the plunger 52 through the hole 48 as previously described. However, should the main fuse element 53 blow, while residual current is flowing through the ionized remains thereof between junction point 65 and junction point 58, the contact 72 will be retained in its open state against the biasing force of the spring 76 and no electrically conducting path will existbetween wire 68 and the wire 50. However, as the current approaches zero, the magnetic strength of the electromagnet 62 will decrease to such a point that the force of the spring 76 will actuate the contacts or terminals 72 and 74 to close or make electrical contact and thus current will flow through the wire 50 whereupon the wire 50 will immediately melt allowing the plunger v42 to be actuated upwardly, as viewed in FIG. 3 by the discharging action of the spring 44 to thereby protrude through the hole 38 and thereby actuate causing an operation of a tripping mechanism as previously described. It is to be noted that the movement of the plunger 42 by the spring 44 is limited by the plunger housing 40 which is disposed to be engaged by the enlarged portion of the plunger 42 and act as a stop. It will be noted that the plunger 42 will not be caused to move until two conditions have been satisfiedQFirst, the current flowing through the main fuse element 53 or remains thereof must be zero or very close to that value before wire 50 is fused to thereby cause the plunger 42 to move, and second, the fuse element 53 must have begun to blow. Consequently, when the ferrule 24 swings away from the upper fuse latching mechanism 16, as shown in FIG. 1, insufficient current will be flowing to cause an arc therebetween.

Referring now to FIG. 4, another embodiment of the invention is shown. In this case, fuse housing assembly 19, which comprises the fuse housing assembly 20 and the electrically conducting end ferrule or terminal 24,

supports or contains therein a plunger mechanism or device 40 having the previously described plunger 42 and spring 44. In addition, the previously described high tensile strength wire 50 is disposed at point 52 on plunger 42 to prevent it from moving toward hole 38. The wire 50 is constrained from slipping by the electrically insulating disk shaped or similarly shaped member 54 which also acts as a spring seat for the spring 44. In this embodiment, all of the electrical current between the main fuse element 53 and the upper end ferrule 24 flows through an electromagnetic means 80 which comprises an insulated electrical coil or winding 82 which is electrically connected between the junction 84 of the upper end ferrule 24 and another junction 86 of the previously described wire 50 and the main fuse element 53. Main fuse element 53 and electrically conducting wire 50 are connected at the other ends thereof I to a junction point 58 and thence to the other end ferrule (not shown) to normally complete an electrically conducting circuit between both ends or ferrules of the fuse unit or assembly 19. A hinged magnetic shutter means 88 is disposed adjacent to one portion or end of the electromagnetic means or electromagnet 80. The hinged shutter means 88 is spring loaded by means of a biasing spring 90 between the shutter 88 and one wall of the ferrule 24. In one embodiment of the invention, although this is not limiting, the shutter 88 may be pivoted or rotated about a pivot point 92 so that it may swing freely thereabout in a rotating direction 93 under proper conditions. The movable shutter member 88 normally retains or' restrains the plunger 42 in the region 94 in engagement with the plunger 42. Consequently, even if the plunger 42 were otherwise free to move toward hole 38, as previously described, the blocking action of the shutter 88 would prevent that movement from occurring under normal conditions.

During normal conditions, electrical current flows through the end ferrule 24 into the junction point 84 through the electrically conducting insulated coil of wire 82 and into the junction 86, from which it essentially flows to the main fuse element 53, to the junction point 58 and to the other end ferrule of the fuse unit 19 thus providing electrical continuity between the end ferrules of the fuse unit. The wire 50 is disposed in parallel circuit relationship with the main fuse element 53 at the junction points 86 and 58. However, the resistance of this wire is so high with respect to that of the main fuse element 53 that minimal current will flow through it during normal operating conditions and most of the current will flow through that branch of the parallel circuit comprising the main fuse element 53. However, if sufficiently high current flows to cause main fuse element 53 to fuse or open, the resistance of that path will increase rapidly thus forcing a greater amount of current to flow through the wire 50 and causing it to quickly melt. As described previously, the electrical current may not be completely interrupted and may continue to flow through the portion of the fuse unit in which fuse element 53 had previously existed. This may be a plasma type of current flow typical of most electric arcs. While such current continues to flow, it is undesirable to allow the plunger 42 to be actuated through hole 38 to cause a tripping action. This is because the arc, which was described with respect to the previous figures, may be detrimental. However, as long as current is flowing through the disintegrated portion of the main fuse element '53, electrical current must by necessity also be flowing through the coil 82 of the electromagnet thus energizing that electromagnet to maintain shutter 88 in a position to block plunger 42, even though the restraining wire 50 has been destroyed thus pennitting spring 44 to actuate plunger 42 to move toward hole 38. It is not until the current flowing through coil 82 and consequently through the disintegrated portion of fuse element 53 decreases to a sufficiently low value to allow the magnetic force to be less than the force of the loaded spring that the shutter 88 is quickly driven by spring 90 to pivot about a pivot point 92 in a direction 93 to thereby allow it to clear the plunger 42 to thereby allow the plunger 42 to move axially into opening 38 thereafter to cause a tripping action such as previously described.

It is to be understood that the fuse assembly shown in FIG. 1 is not limiting and the various embodiments of the present invention may be used in any type of dropout fuse or apparatus or fuse assembly which requires a tripping action to be initiated from within the fuse unit housing. The teachings of the present invention may be used in any apparatus where the mere protrusion of a plunger out the end or any outer surface of the fuse housing is required or necessary. It is also to be understood that the materials of the components of the plunger assembly and the various other assemblies associated therewith are not limited to those previously described. For example, the side walls of the plunger assembly 40 may be electrically conducting or electrically insulating. The plunger 42 may be electrically conducting or electrically insulating. However, the support disk or member 44 should be electrically insulating unless suitable insulating materials are provided for those portions of the wire 50 which come in contact therewith or else the portion of the wire 50 inside the plunger housing may be shorted out. It is also to be understood that the embodiment of FIG. 4, in which the shutter 88 is shown as pivoting is not limiting, a shutter may ,move rectilinearly or translationally rather than rotationally if desired. It is also'to be understood that the springs 76 and 90 may be used to bias the various apparatus to which they are attached in the opposite directions than those which are described with respect to FIGS. 3 and 4, provided that the electromagnet is suitably energized to cause the apparatus or parts involved to move in such a manner as to free the plunger as the electrical current in the main fuse element 53 approaches zero. It is also to be understood that the apparatus of this teaching may be used in either high voltage or low voltage fuses or high current or low current fuses. It is also to be understood that in some embodiments of the invention, the teachings of the embodiments of FIGS. 3 and 4 may be combined to provide a latch release means of even greater flexibility and capabilities. It is also to be understood that the fuse schematically or diagrammatically represented by 53 may comprise a plurality of fuse elements in series or parallel or any combination thereof.

It is also to be understood that the invention is particularly adapted to current limiting fuses in which the fuse housing 20 may contain pulverulent material such as quartz sand which may be fused into a fulgerite upon the blowing of the fuse element 53 and in which the fuse element 53 comprises at least one region of reduced cross section for current limiting purposes upon fusing.

The teachings of the'present invention have many adhousing or unit disclosed is not allowed to rotate free of its latching assembly until detrimental electrical current therethrough which has caused the fusing action is reduced to zero or interrupted. This prevents the for mation of an are between ferrule and latch which is undesirable. Another advantage lies in the fact that the triggering mechanism is contained entirely within the body of the fuse housing rather than within the stationary or latch portion of the overall fuse assembly. Another advantages lies in the fact that the fusing action associated with the main fuse element 53 is not deterred by the operation of the latching plunger mechanism.-

What we claim is:

. l. A fuse structure, comprising:

' a. a fuse housing having spaced terminals thereon, one of which is adapted to be alternately held by or released from a combination line contact and latch means and'a meltable fuse element connected betweensaid terminals and enclosed by said housing to conduct electrical current;

b. a latch actuator for actuating said latch means to unlatch said one terminal;

c. a first means disposed within said housing for sens ing the presence of substantially zero electrical current in said fuse element flowing between said terminals within said housing and preventing said latch actuator from actuating an unlatching operation whenever said electrical current is not substantially zero; and

d. a second means disposed within said fuse housing for sensing when said fuse element has melted and normally preventing said latch actuator from actuating an unlatching operation until said fuse element has melted. 7

2. The combination claimed in claim 1 wherein said latch actuator comprises a plunger loaded by a spring and adapted to move through a portion of said one terminal during an unlatching operation to actuate the release of said one terminal from said latch means.

3. The combination as claimed in claim 2 wherein said second means comprises an additional fusible element which is normally attached to said plunger to prevent it from being moved under the influence of said spring, said additional fusible element beingconnected in circuit relationship between said terminals to fuse when said electrical current flowing between said terminals achieves a predetermined value and said firstmentioned fuse element melts to thereby permit said plunger to move under the influence of said spring and actuate the release of said one terminal from said latch means. j

4. The combination as claimed in claim 3 wherein said'f rst means comprises:

1. an electromagnet having an electrical winding through which magnetizing current flows, one end of said winding being connected to one of said fuse terminals, the other end thereof being connected to said first-mentioned fuse element so that any current which flows between said terminals through said fuse element also flows through said electrical winding for energizing said electromagnet; and

2. a blocking means, at least a portion of which is magnetic and disposed proximate to said electromagnet and being actuable between first and second orientations with respect to said plunger to prevent said plunger from moving when current is flowing in said winding and said blocking means is disposed in said first orientation independent of the operating status of said additional fusible element and also cooperating with said plunger to allow said plunger to move when said winding is substantially deenergized and said blocking means is in said second orientation, said blocking means being actuable from said first orientation to second orientation whenthe current flowing in the winding of said electromagnet decreases to a predetermined value. 5. The combination as claimed in claim 3 wherein said first means comprises:

1. an electromagnet having an electrical winding through which energizing current flows, one end of said winding being connected to one of said fuse terminals, the other end thereof being connected to said first-mentioned fuse element so that said current which flows between said terminals through said fuse element also flows through said electrical winding for energizing said electromagnet;

separable electrical contacts, one of said contacts being spring loaded and disposed proximate to. said electromagnet, said last-mentioned contact being actuable between open or closed positions with respect to the other contact, said other of said contacts being electrically connected to said additional fusible element, the other of said contacts being connected to said electromagnet winding, said contacts being open to prevent current flow through said additional fusible element and to prevent said plunger from moving whenever said current through said winding and said first-mentioned fuse element is not c. a fuse housing having spaced terminals thereon,

one of which is adapted to be held by or released from said latching means, and a fuse element connected between said terminals and enclosed by said housing to normally conduct electrical current; d. a latch actuator for actuating the unlatching of said one terminal from said latching means; a first means disposed within said housing for sensing the presence of substantially zero electrical current flowing through said fuse element between said terminals within said housing and preventing said latch actuator from actuating an unlatching whenever said electrical current is not substantially zero; and g f. a second means disposed within said fuse housing for sensing when said fuse element has melted and normally preventing said latch actuator from actuating an unlatching operation until said fuse element has melted.

7. The combination claimed in claim 6 wherein said latch actuator comprises a plunger loaded by a spring and adapted to .move through an opening in said one terminal during an unlatching operation to actuate the release of said one terminal from said latching means.

8. The combination as claimed in claim 7 wherein said second means comprises a second fusible element which is normally attached to said plunger to prevent it from being moved by said spring, said second fusible element being connected in circuit relationship between said terrninals to fuse when said electrical current flowing between said terminals achieves a predetermined value and said first-mentioned fues element melts, to thereby permit said plunger to actuate the release of said one terminal from said latching means.

9. The combination as claimed in claim 8 wherein said first means comprises:

1 an electromagnet having an electrical winding through which energizing current flows, one end of said winding being connected to one of said 'fuse terminals, the other end thereof being connected to said first-mentioned fuse element so that any current which flows between said terminals through said fuse element also flows through said electrical winding for energizing said electromagnet;

2. a blocking means capable of being in either a first or second orientation with respect to said plunger cooperating with said plunger to prevent it from moving when said shutter means is disposed in said first orientation independent of the operating status of said second means fusible ma- 10. The combination as claimed in claim 8 wherein said first means comprises:

1. an electromagnet having an electrical winding through which energizing current flows, one end of said winding being connected to one of said fuse terminals, the other end thereof being connected to said main fuse element so that said current which flows between said terminals of said fuse element also flows through said electrical conductor for energizing said electromagnet;

2. spaced electrical contacts one of which is spring loaded and disposed proximate to said electromagnet said contact being capable of being in either an open or closed position, one of said contacts being electrically connected to said second means fusible material, the other of said contacts being connected to a portion of said electromagnet conductor, said contact being opened to prevent said plunger from moving whenever said current between said conductor is not essentially zero amps, said plunger only moving to unlatch said terminal from said housing when said current between said terminal is essentially zero to thereby cause said contact to join and said second means fusible material has blown. 

1. A fuse structure, comprising: a. a fuse housing having spaced terminals thereon, one of which is adapted to be alternately held by or released from a combination line contact and latch means and a meltable fuse element connected between said terminals and enclosed by said housing to conduct electrical current; b. a latch actuator for actuating said latch means to unlatch said one terminal; c. a first means disposed within said housing for sensing the presence of substantially zero electrical current in said fuse element flowing between said terminals within said housing and preventing said latch actuator from actuating an unlatching operation whenever said electrical current is not substantially zero; and d. a second means disposed within said fuse housing for sensing when said fuse element has melted and normally preventing said latch actuator from actuating an unlatching operation until said fuse element has melted.
 2. The combination claimed in claim 1 wherein said latch actuator comprises a plunger loaded by a spring and adapted to move through a portion of said one terminal during an unlatching operation to actuate the release of said one terminal from said latch means.
 2. spaced electrical contacts one of which is spring loaded and disposed proximate to said electromagnet said contact being capable of being in either an open or closed position, one of said contacts being electrically connected to said second means fusible material, the other of said contacts being connected to a portion of said electromagnet conductor, said contact being opened to prevent said plunger from moving whenever said current between said conductor is not essentially zero amps, said plunger only moving to unlatch said terminal from said housing when said current between said terminal is essentially zero to thereby cause said contact to join and said second means fusible material has blown.
 2. a blocking means capable of being in either a first or second orientation with respect to said plunger cooperating with said plunger to prevent it from moving when said shutter means is disposed in said first orientation independent of the operating status of said second means fusible material, and also cooperating with said plunger to allow said plunger to move when said shutter means is in said second orientation, said shutter means being actuable from one of said first or second orientations to the other of said first or second orientations by a significant change in the status of said current flowing in the conductor of said electromagnet.
 2. separable electrical contacts, one of said contacts being spring loaded and disposed proximate to said electromagnet, said last-mentioned contact being actuable between open or closed positions with respect to the other contact, said other of said contacts being electrically connected to said additional fusible element, the other of said contacts being connected to said eleCtromagnet winding, said contacts being open to prevent current flow through said additional fusible element and to prevent said plunger from moving whenever said current through said winding and said first-mentioned fuse element is not essentially zero amperes, said plunger only moving to actuate the unlatching of said terminal from said latch means when said current between said terminals is essentially zero to thereby cause said contacts to close and said additional fusible element has blown.
 2. a blocking means, at least a portion of which is magnetic and disposed proximate to said electromagnet and being actuable between first and second orientations with respect to said plunger to prevent said plunger from moving when current is flowing in said winding and said blocking means is disposed in said first orientation independent of the operating status of said additional fusible element and also cooperating with said plunger to allow said plunger to move when said winding is substantially deenergized and said blocking means is in said second orientation, said blocking means being actuable from said first orientation to second orientation when the current flowing in the winding of said electromagnet decreases to a predetermined value.
 3. The combination as claimed in claim 2 wherein said second means comprises an additional fusible element which is normally attached to said plunger to prevent it from being moved under the influence of said spring, said additional fusible element being connected in circuit relationship between said terminals to fuse when said electrical current flowing between said terminals achieves a predetermined value and said first-mentioned fuse element melts to thereby permit said plunger to move under the influence of said spring and actuate the release of said one terminal from said latch means.
 4. The combination as claimed in claim 3 wherein said first means comprises:
 5. The combination as claimed in claim 3 wherein said first means comprises:
 6. A dropout fuse assembly comprising: a. a fuse assembly support means with spaced electrically conducting supports adjacent the ends thereof; b. a fuse latching means disposed on one of said supports for retaining a fuse housing in place in said fuse assembly; c. a fuse housing having spaced terminals thereon, one of which is adapted to be held by or released from said latching means, and a fuse element connected between said terminals and enclosed by said housing to normally conduct electrical current; d. a latch actuator for actuating the unlatching of said one terminal from said latching means; e. a first means disposed within said housing for sensing the presence of substantially zero electrical current flowing through said fuse element between said terminals within said housing and preventing said latch actuator from actuating an unlatching whenever said electrical current is not substantially zero; and f. a second means disposed within said fuse housing for sensing when said fuse element has melted and normally preventing said latch actuator from actuating an unlatching operation until said fuse element has melted.
 7. The combination claimed in claim 6 wherein said latch actuator comprises a plunger loaded by a spring and adapted to move through an opening in said one terminal during an unlatching operation to actuate the release of said one terminal from said latching means.
 8. The combination as claimed in claim 7 wherein said second means comprises a second fusible element which is normally attached to said plunger to prevent it from being moved by said spring, said second fusible element being connected in circuit relationship between said terminals to fuse when said electrical current flowing between said terminals achieves a predetermined value and said first-mentioned fues element melts, to thereby permit said plunger to actuate the release of said one terminal from said latching means.
 9. The combination as claimed in claim 8 wherein said first means comprises:
 10. The combination as claimed in claim 8 wherein said first means comprises: 